1. Field of the invention
The present invention relates to a torque converter for an automatic transmission having a lock-up clutch, more specifically to a slip control system for the lock-up clutch.
2. Description of the Prior art
In a field of a torque converter incorporated to an automatic transmission of an automotive vehicle, there has been proposed a lock-up clutch for providing a direct connection between engine output member and input member of the transmission under a predetermined vehicle operating condition wherein there occurs no amplification of a torque, or no change in torque from an engine so that a fuel consumption performance of the vehicle is improved.
When the lock-up clutch is engaged, the torque from the engine is transmitted bypassing the torque converter to the transmission through the lock-up clutch. Under this operation, it is advantageous in that the fuel consumption performance can be improved because there is no energy loss caused by a fluid friction in the torque converter. On the other hand, it is disadvantageous in that the torque amplification and a shock absorbing function of the torque converter cannot be obtained to thereby deteriorate a riding comfort.
In view of the above characteristics of lock-up clutch of the torque converter, there has been proposed a slip control for the lock-up clutch wherein the lock-up clutch is controlled to produce a certain slippage of the lock-up clutch or a rotation speed difference between an input and output members of the toque converter to avoid the above disadvantage.
Japanese Patent Public Disclosure No. 57-33253 (corresponding to U.S. Pat. No. 4,468,988 issued on Sept. 4, 1984) laid open to the public in 1982, discloses a slip control system for a lock-up clutch of a torque converter wherein respective rotation speeds of an input and output members of the torque converter are detected and a hydraulic pressure for controlling the lock-up clutch is controlled for converging a rotation speed difference between the input and output members to a predetermined value by means of a feedback control. U.S. Pat. Nos. 4,580,671, 4,618,041, 4,669,441, 4,687,083 disclose conventional transmission control systems respectively.
In the slip control of the lock-up clutch, an engaging force of the lock-up clutch or the amount of the slippage thereof is usually determined based on the vehicle operating condition, such as an engine load, an engine speed, and/or a vehicle speed.
In the automatic transmission provided with the slip control system for the torque converter, when an operation of the transmission changes from a non-slip control condition wherein the engaging force of the lock-up clutch is controlled not to allow the slippage thereof substantially (a lock-up control condition) or no engaging force is exerted in the lock-up clutch, that is, the lock-up clutch is fully disengaged (a converter control condition ) to a slip control condition wherein the engaging force of the lock-up clutch is controlled to allow a certain slippage of the lock-up clutch, there produces a certain torque shock undesirably. The torque shock may also occurs when a change in the control for the lock-up clutch between the lock-up control condition and the converter control condition.
In order to reduce the torque shock in a transient condition from the non-slip control condition to the slip control condition, when the transient condition occurs in the case where the control for the lock-up clutch is changed from the converter control condition to the slip control condition or to the lock-up control condition through the slip control condition, the engaging force of the lock-up clutch is usually controlled at a value which provides a relatively large slippage of the lock-up clutch at a first stage and then is controlled to be decreased gradually toward a target value (including a value of no slippage). On the other hand, when the transient condition occurs in the case where the lock-up control condition is transferred to the slip control condition or to the converter control condition through the slip control condition, the engaging force is set at a smaller value than the final target value and is increased gradually toward the target value (including the converter control condition).
It should however be noted that the torque shock caused by the change from the non-slip control condition to the slip control condition depends on the vehicle operating condition just before the slip control condition is initiated. Therefore, even through the engaging force of the lock-up clutch is commonly controlled to ease the torque shock for the respective transient conditions, there might occur a problem that the torque shock cannot be removed effectively in some vehicle operating conditions at the time of a transition between the non-slip control condition and the slip control condition.
A longer period transient control would be effective for easing the torque shock.
However, it is impossible for the control to accomplish a responsive slip control.